1. Field of the Invention
The present invention relates to a laser recording apparatus for forming an image by exposing a photoconductor to a laser beam for scanning and particularly it relates to an image recording apparatus with a variable magnification which changes a magnification of a binary image finely by gradual steps in the main scanning direction.
1. Description of the Background Art
Such a recording apparatus known in the prior art is for example a laser plotter of a cylindrical scanning type for use in art work of print connection patterns and the like.
An image recording unit of this laser plotter comprises, as shown in FIG. 1, a rotary drum 16 supporting a sheet of film 14, a main scanning drive device (not shown) for rotating the rotary drum 16 in the main scanning direction, an image forming head 20 having a laser beam oscillator, and a subscanning device (not shown) for moving the head 20 in a subscanning direction.
The image forming method of this laser plotter is as follows.
The main scanning drive device rotates the rotary drum 16. The image forming head 20 modulates on and off a laser beam in accordance with the image formation data. At the same time, the image forming head 20 moves in the subscanning direction. As a result, a line in the main scanning direction containing a series of dots is formed on the film 14, whereby a binary image as a set of lines in the main scanning direction is recorded.
Generally, various methods are known as methods of changing a size of an image to be recorded in such a scanning type recording apparatus. Among those, the following two methods are of special interest to the present invention. The first method is a method in which an output is provided through calculation for increasing or decreasing the number of recorded dots according to a variable magnification, based on data to be recorded. The second method is a method in which a basic scanning unit (a dot) for forming an image is expanded or reduced in a manner of changing the one dot's irradiation time of a laser beam. For this purpose, the wavelength of a clock signal serving as a reference for controlling the one dot's irradiation time of the laser beam is changed.
According to the first method (as disclosed for example in Japanese Patent Laying-Open No. 65601/1979), recording with a variable magnification is carried out by skipping some dots or adding interpolative dots according to the change of the magnification. However, in this method, part of the entire data to be outputted is skipped or some interpolative dots are added to the entire data to be outputted. In consequence, the definition of the output image is not preferable. In addition, it is impossible in this method to reduce or expand the image by an area smaller than one standard dot.
On the other hand, it is sometimes necessary to compensate for an error of the magnification dependent on a finishing precision of a rotary drum or to correct an error of a magnification caused by a difference in thickness of a film. In such cases, it is necessary to set a magnification change ratio by gradual steps, for example, of about .+-.0.001%. For those purposes, the above mentioned second method is adopted.
According to the second method, the size of one dot is expanded or reduced in the main scanning direction according to the magnification change ratio so that the size of the image can be changed. An apparatus of interest to the present invention is proposed in Japanese Patent Laying-Open No. 161869/1986. This apparatus is capable of adjusting the magnification change ratio with a precision of 0.001%. FIG. 2 is a block diagram showing the operation principle of the apparatus.
Referring to FIG. 2, the conventional variable magnification image recording apparatus comprises: an image recording unit 2 for scanning an image on photosensitive material such as a film and forming the image; a reference clock signal generator 5 for generating a reference clock signal; a high-frequency clock signal generator 3 connected to the reference clock signal generator 5, for generating a clock signal having a high frequency; a 1/L frequency divider 8 connected to the high-frequency clock signal generator 3, for applying frequency division to the high-frequency clock signal to generate an image forming clock signal; an image data storing unit 10 for storing image information and outputting an image formation data; and an image data processing unit 12 connected to the image data storing unit 10 and an output of the 1/L frequency divider 8, for outputting an image recording signal.
FIG. 1 is a detailed block diagram of the apparatus shown in FIG. 2. Referring to FIG. 1, the image recording unit 2 comprises: a rotary drum 16 for supporting a film 14; drum rotating means (not shown) for rotating the rotary drum in the main scanning direction; a rotary encoder 18 provided on a rotating shaft of the rotary drum 16, for outputting a reference clock signal in response to the rotation of the rotary drum 16; an image forming head 20 having a laser beam oscillator for forming an image composed of a plurality of dots on the film 14 by irradiation of a laser beam; and subscanning means (not shown) for moving the image forming head 20 in the subscanning direction.
The high-frequency clock signal generator 3 comprises a phase-locked loop (PLL) circuit 4 connected to the rotary encoder 18, for multiplying a frequency of the reference clock signal, and a higher-frequency clock signal generator 6, for generating a clock signal of a higher frequency than that of the output signal of the PLL circuit 4.
The PLL circuit 4 comprises a phase comparator 22, a low-pass filter 24, and a voltage control oscillator 26 connected successively with an output of the rotary encoder 18 and further comprises a 1K frequency divider 28 provided in a feedback loop from an output of the voltage control oscillator 26 to the phase comparator 22.
The higher-frequency clock signal generator 6 comprises a 1/m frequency divider 30 and a 1/n frequency divider 32, both connected to an output of the PLL circuit 4, second and third PLL circuits 34 and 36 connected to outputs of the frequency dividers 30 and 32, respectively, and a frequency mixer 38 connected to output of the third PLL circuit 36 and to the second PLL circuit 34.
The second PLL circuit 34 comprises a phase comparator 40, a low-pass filter 42 and a voltage control oscillator 44 connected successively with an output of the 1/m frequency divider 30, and a 1/M frequency divider 46 provided in a feedback loop from the frequency mixer 38 to the phase comparator 40. An output of the voltage control oscillator 44 is connected to the 1/L frequency divider 8 and the frequency mixer 38.
The third PLL circuit 36 comprises a phase comparator 48, a low-pass filter 50 and a voltage control oscillator 52 connected successively with an output of the 1/n frequency divider 32 and further comprises a 1/N frequency divider 54 provided in a feedback loop from the voltage control oscillator 52 to the phase comparator 48. Each of the characters m, n, K, L, M, and N represents a natural number and the represented numbers can be changed.
Referring to FIG. 1, operation of the conventional apparatus will be described. The rotary drum 16 is rotated by the drum rotating means in the main scanning direction. The rotary encoder 18 generates a signal having a frequency fi in response to the rotation of the rotary drum 16. The PLL circuit 4 multiplies the frequency of the reference clock signal inputted by the rotary encoder 18 by K and generates a clock signal having a frequency K.multidot.fi.
If it is not necessary to change the magnification finely by small steps, the output of the PLL circuit 4 may be connected directly to the 1/L frequency divider 8. In this case, the higher-frequency clock signal generator 6 is not necessary and the frequency fo of the clock signal outputted from the 1/L frequency divider 8 is as follows: EQU fo=K.multidot.fi/L
where K and L are both natural numbers. It is known that in order to accurately adjust the frequency fo with a change amount of about 0.001%, K needs to be about 10.sup.5. However, if K is 10.sup.5 and fi is about 100 kHz, the frequency of the clock signal outputted by the PLL circuit 4 becomes too high, causing the operation of the PLL circuit 4 to be unstable. Therefore, it is known that if the output of the PLL circuit 4 is connected directly to the 1/L frequency divider 8, a magnification change ratio cannot be set with the change amount of about 0.001%.
The higher-frequency clock signal generator 6 outputs a signal having a high-frequency necessary for obtaining a desired magnification change ratio while the respective frequencies of the output clock signals of the PLL circuits 4, 34 and 36 are prevented from being too high. The 1/L frequency divider 8 divides, by L, the frequency of the signal outputted from the higher-frequency clock signal generator 6 and outputs an image forming clock signal having the frequency fo. The image data processing unit 12 outputs an image recording signal having two values dependent on image data, synchronizing the image forming signal inputted from the image data storing unit 10 with the image forming clock signal. The image forming head 20 receives the image recording signal and modulates on and off a laser beam, so that an image composed of dots is recorded on the film 14. The film 14 rotates together with the rotary drum 16 in the main scanning direction. The image forming head 20 is moved with a high precision in the subscanning direction by the subscanning means. Thus, the binary image is formed on the film 14.
The operation of the higher-frequency clock signal generator 6 will be described in more detail. The 1/m frequency divider 30 outputs a clock signal having a frequency f1=K.multidot.fi/m. The 1/n frequency divider 32 outputs a clock signal having a frequency f2=K.multidot.fi/n. The PLL circuit 36 outputs a clock signal having a frequency fo2=N.multidot.K.multidot.fi/n. The PLL circuit 34 has an output clock signal of frequency fol. The frequency mixer 38 outputs a clock signal having a frequency f3=fol-fo2 and supplies it to the 1/M frequency divider 46. A clock signal outputted from the 1/M frequency divider 46 has a frequency f3/M.
The PLL circuit 34 adjusts the output frequency fol so that the frequencies of the two signals inputted to the phase comparator 40 may be equal. Accordingly, the following equation is obtained. ##EQU1##
As a result, the following equation is established. ##EQU2##
Consequently, the frequency fo of the synchronizing signal outputted from the 1/L frequency divider 8 is as follows. EQU fo=K.multidot.(M/m+N/n).multidot.fi/L
The frequency dividing factor of the 1/M frequency divider 46 is changed from M to M+1 and the frequency dividing factor of the 1/N frequency divider 54 is changed from N to N-1 at the same time. At this time, the frequency fo of the image forming clock signal is changed with a unit change ratio: EQU .delta.=(n-m)/(nM+mN)
Particularly, in view of stable operation of the PLL circuits 34 and 36, the numbers m, n, M and N are selected to be about 10.sup.2 to 10.sup.3. With a condition of 1.ltoreq.n-m.ltoreq.10, the ratio .delta. can be selected to be about 10.sup.-5. More specifically, the frequency change ratio can be adjusted by steps of 10.sup.-5. In other words, it can be set with a unit of 0.001%.
In the image recording unit 2, the laser beam is modulated on and off in response to the image recording signal, more particularly, synchronously with the image forming clock signal because the image recording signal is synchronized with the image forming signal. Accordingly, the size of the standard dot in the main scanning direction is proportional to the reciprocal number of the frequency (more particularly, the wavelength) of the image forming clock signal. Since the frequency of the image forming clock signal can be adjusted with a small change amount, the size of the standard dot for image formation can be also adjusted with a small change amount. In this case, the sizes of all the dots contained in an image are equal and the size in the main scanning direction is changed exactly by an amount corresponding to the change of the magnification, compared with the dot size in the case of the magnification of 1.
However, the conventional image recording apparatus with a variable magnification has the below described disadvantages. One of the disadvantages is that operation for determining conditions for obtaining a desired magnification is complicated. Variables to be set are at least as many as six. Therefore, in general, there is no effective method for defining the most suitable combination of the variables. As a result, it is necessary to take account of a large number of combinations of the variables by setting conditions such as the frequency dividing factors of the respective frequency dividers.
Another disadvantage is the complicated work for setting the circuits to obtain a desired magnification. The elements which need to be adjusted for obtaining the desired magnification are dispersed in the circuits. Therefore, the circuit adjusting work for obtaining the desired magnification is troublesome.
A further disadvantage is that the circuit construction is complicated. The circuitry has many components. In addition, a desired magnification cannot be obtained unless all the components operate correctly. Thus, the maintenance efficiency of the apparatus would be likely to be lowered.